trinityamps.com

Hello...
This is my first post on this forum. I'm on my 4th amp...3 of which have been valve junior conversions using pcbs from guitaramplifierpcbs....PlexiSE, Tweed 5E3, and Baby Will. These have gone together with no problems at all, but this amp is giving me some problems...Hoping someone can help me out here...
I have a Superlite-II PCB that I just put together from GuitarAmplifierPCBs. I am using the Trinity 18 watt transformer set and a GZ34 rectifier. I have followed the BOM to the "t' (except for the transformers) and checked all component values back to the schematic, as well as checked all solder connections etc. However, the cathode voltage seems to be on the low side. I have no reference values for this board because Terry recently passed away before he was able to put together a build manual. But here's what I have (all values VDC):

B+ = 335
after R4: 303 (B)
after R5: 286 (C)
after R6: 277 (D)

My voltage across R28 (150R 5W cathode resistor) is 6.64

V3:
pin 3: 6.6
pin 7: 308
pin 9: 301

V4:
pin 3: 6.6
pin 7: 302
pin 9: 300

Not sure what to do here...the cathode resistor is really stumping me. Any guidance you can offer is greatly appreciated!
Here's a link to the schematic
http://guitaramplifierpcbs.com/Documents/SuperLite%20II%20v2.7%20Schematic%20v3.pdf

Thanks!
Bruce

Well this is kind of embarrassing, but I decided to shut the amp down and check the actual resistance of my cathode resistor....it measured 37R!!!! Resistor is labeled as 150R.

So when I calculated the bias using these numbers, it looks like I was running the EL84's at 26 watts/85mA. I'm assuming these tubes are pretty much shot now.

Anyways...now I gotta find a replacement. I might have something in the ballpark to get in there just to check things out.

This is really strange...I removed the 150R resistor that I thought was "bad", measured it and sure enough it read 150R. I put in a new resistor that measured 150R, and took a reading across the resistor after putting it in the board, and it reads 37R. I checked the tracings to see if there were any shorts of any kind or irregularities in the tracings, and everything looks fine.

Any ideas?

That's normal when you have a capacitor attached in parallel.

To get an accurate reading you need to remove the resistor from the circuit (like you did).

_________________
http://pdfelectronics.com

If you have a short to ground on your filament cct it would be about 37r.
R28/R1/R2
And I think the cathode bypass cap should be more like 220mf not 2200mf
Did you connect the centre tap of the heater cct. That would explain the 37r reading.

Its not normal, try it yourself. connect the leads of a resistor and polarized cap. The cap charges and then the resitance reading will be the value of the resistor.

_________________
Fantasy Noise is what my shop of dreams is called and was the only way to beat my addiction, like many here I suffer from G.A.S.


Tele's make the best noise

I have the center tap of the 6.3VAC heaters attached to the star ground on the chassis.....should I remove this, heatshrink it and tie it off? Sorry...my "newbie-ness" is really showing here.

I removed the resistor and also the 2200mfd cap from the board, connected them in parallel using a tagboard on my bench and took a reading...it was 148R. Also, with the cap removed and the 150R resistor in place on the board, it still measures 37R on the board. So the cap being in parallel with the resistor is not the culprit.

As far as the 2200mfd...that's what was on the schematic, so that's what I went with. I went back and looked at Terry's schematics for the Baby Will (Lite IIb), Superlite, and Superlite-II (this build) and the cap values go from 47, 470, then 2200mfd, respectively.

thanks for the help so far guys!

Bruce

Remove the tubes, lift the CT on the filaments and check voltages are in the ballpark. Then try it with tubes in. check voltages and operation.
Schematic shows a 6.3 filament supply with no center tap and no ground in that half the cct which means the 100r(R1, R2) are no longer parallel to the cathode resistor.
Good luck

_________________
Fantasy Noise is what my shop of dreams is called and was the only way to beat my addiction, like many here I suffer from G.A.S.


Tele's make the best noise

that took care of it....
Where I think I went wrong was that the new pcb, the Superlite-2, called for a power supply without a heaters centertap, and an artificial center tap was built into the board. Since I decided to go with the Trinity transformers and a tube rectifier, I went back to the superlite-1 schematic (which had different wiring diagrams options for different transformers and rectifiers), and I used that as a basis for what I was wiring in the Superlite-2. I notice now that the Superlite-1 schematic specifies that an artificial CT for the heaters must be employed "off-board" if using a transformer without a heaters ct.

I'll chalk this up to a lesson learned. Thanks for the help. Now I need to bring my rail voltages down a little.

Bruce

FYI, your voltages will probably be too high with a GZ34, so you may need to insert some zener diodes to get the voltage down to around 335V-ish, or else use a VVR/VRM.

I'm also not really sure that the changes Terry (RIP) made to my SuperLite design went in the right direction. You may like to try reversing the resistor and cap changes in the Lite II (eg split load resistor on first stage and no cathode cap on stage 2) to take it back to the Lite I preamp, and see which way you prefer it.

_________________
Great tones come in small glass jars!

Zaphod...
Thanks for the info!
I was researching a little on the Zeners already, and was thinking of running a few in series with the ct on a tag strip and just using the center lug on the tag strip as a ground on the chassis PT bolt. Can't really find zeners around here....I was looking at getting a few 10V/5W and 12-15V/5W to see what would get me in the ball park. I guess its time to place an order in to mouser. Any specific zeners that are recommended? I think I just need to make sure I get ones that can handle at least 5W dissipation?

I will definitely try your suggestions for the preamp. One thing I've found with these PCBs...as nice as they are for novices like myself, they sure are a royal pain to do component experimentation on. This will definitely be the last PCB amp I do. In the Lite-2 PCB, there is (vacated) space for the cathode cap (C8) on the board...should I use the 1uf/50V as shown in the SuperLite-1 PCB schematic?

thanks again!
Bruce

Yes, I would recommend the 1uF cathode cap. It helps increase gain and as well as limiting bass. I was never too convinced at the explanation Terry (RIP) gave for why he changed the 18W SuperLite's preamp design.

For safety reasons you need to limit the power dissipated by each zener. So a string of 6V 5W zeners is half as likely to blow than half the number of 12V 5W zeners.

_________________
Great tones come in small glass jars!

Got the cap and some zeners on order from Mouser.
Thanks...I'll post back when I get the voltages down some. Will also evaluate C8 in/out.
Bruce

ok...
put in a string of four 9.6v/5W zeners in line with the ct to ground. Here's where I'm at:

b+ 330v (previous was about 365v)

V3
pin 3 = 12
pin 7 = 322
pin 9 = 309

V4
pin 3 = 12
pin 7 = 318
pin 9 = 309

Actual VDC measured across pins 7 and 3:
V3: 310
V4: 306

I am calculating 12W dissipation per tube, and about 38mA.

Not sure why V4 pin 7 is running lower than V3. I tried a matched set of JJ's and also a matched set of new production Mullards and got the same results.

But the zeners seem to be doing the job. Any advice on where to troubleshoot the voltage disparities between V3 and V4?

Thanks
Bruce

Probably because the inputs are slighly different on pin 2 and the tubes are conducting at a slightly different levels. Is it causing any problems? That should be well withen tolerance.

_________________
Fantasy Noise is what my shop of dreams is called and was the only way to beat my addiction, like many here I suffer from G.A.S.


Tele's make the best noise

Thanks Taz...haven't had a chance to actually play through the amp since bringing the voltages down. I'll try it out this afternoon and get back. will also experiment with C8.

Bruce

Those voltages are perfectly fine. Don't forget that the two halves of the OT primary always have different resistance values.

It's a pity though, that you've spent money on matched tubes. Unmatched ones would have given warmer tones.

_________________
Great tones come in small glass jars!

I have some single JJ's laying around, so I may experiment a little with mix and matching. Thanks for the recommendation!

Bruce